The invention relates to a device and a method for generating digital control signals.
In particular, the invention relates to a device for generating digital control signals which is suitable for pulse-width-modulated driving, for example of a radio-frequency generator. The term radio-frequency generators is understood to mean, in particular, half-bridge-invertors, full-bridge invertors and push-pull invertors which, for example, are used in so-called electronic ballasts for radio-frequency operation of fluorescent lamps or of high-pressure discharge lamps. The frequency of the radio-frequency AC voltage generated by such invertors is tuned to the different operating states of the fluorescent lamps for example by means of a microprocessor and integrated circuits by generation of pulse-width-modulated drive signals for the switching transistors of the invertor, or is used for operating a high-pressure discharge lamp with frequency-modulated AC current in accordance with the published patent application EP 0708579 A1. However, only digital control signals can be generated with the aid of a microprocessor. Accordingly, the frequency of the radio-frequency generator can be altered only in discrete steps. In conventional drive devices, the smallest possible adjustable change in frequency is limited by the operating frequency of the radio-frequency generator. The following example shall clarify these facts in more detail. It is known to perform the driving of the radio-frequency generator with the aid of an auto-reload timer implemented in the microprocessor. The auto-reload timer is essentially a programable counting mechanism which is operated by an oscillator with a predetermined frequency and is loaded with a predeterminable numerical value. If the auto-reload timer is loaded with the numerical value N and if the frequency of the oscillator which predetermines the counting cycle is designated by F, then the auto-reload timer can be used to generate an operating frequency f which is given by the formula f=F/2N. The next highest operating frequency can be achieved with the numerical value Nxe2x88x921. The minimum frequency step xcex94f is therefore xcex94f=f/(Nxe2x88x921). An obtainable operating frequency of 50 kHz and a minimum frequency step of 505 Hz result for N=100 and F=10 MHz. That is insufficient for many applications, in particular for driving radio-frequency generators which are intended to enable radio-frequency operation of fluorescent lamps and finely gradated dimming of the fluorescent lamps.
The object of the invention is to provide an improved device for generating digital control signals. In particular, the advice according to the invention is intended to be suitable for finely gradate pulse-width-modulated driving. Moreover, the object of the invention is to specify an improved method for generating digital control signals.
The invention""s device for generating digital control signals has the following features:
an oscillator, which has a digital output,
a first storage means for storing the value of a digitally coded variable, which has a clock input connected to the digital output of the oscillator, a data input and a data output,
a second storage means for storing the value of a digitally coded variable, which has a clock input connected to the digital output of the oscillator, a signal input and a signal output,
an adder which has two data inputs, a data output and a carry output, where
the first data input of the adder is connected to the data output of the first storage means
the second data input of the adder is connected to the data output of the second storage means,
the data output of the adder is connected to the data input of the second storage means, and
the carry output of the adder is connected to a pulse divider.
The device according to the invention makes it possible to generate digital control signals whose frequency is gradated considerably more finely and which enable correspondingly more accurate pulse-width-modulated driving, for example of a radio-frequency generator. The minimum frequency difference that may be exhibited by two control signals generated by the device according to the invention is limited only by the storage size or word width of the adder and of the two storage means, given a predetermined clock frequency of the oscillator. Each additional bit in the register of the adder halves the minimum frequency difference or the distance between two adjacent frequencies.
The device according to the invention is advantageously equipped with a 16-bit adder and two 16-bit latches. The use of 16-bit components allows the generation of digital control signals whose frequency can already be gradated very finely. If, for example, the clock frequency of the oscillator is 10 MHz, then control signals in the frequency range of from approximately 76.3 Hz to 4.99 MHz can be generated using the abovementioned 16-bit components. The minimum frequency step is approximately 76.3 Hz in the entire frequency range. A T flip-flop is advantageously used as pulse divider in the device according to the invention. It is particularly well suited to converting the carry signal into a pulse-width-modulated control signal, since the output state of the T flip-flop changes upon exactly one edgexe2x80x94upon the occurrence of the rising edge in the present exemplary embodimentxe2x80x94of the carry signal. The device according to the invention may advantageously be designed as part of a microprocessor. It is also possible to implement a plurality of the device according to the invention in a microprocessor in order, for example, to generate a plurality of separate control signals. A microprocessor of this type may advantageously be inserted into a drive device for a radio-frequency generator. The device according to the invention can advantageously be used for frequency control or pulse-width modulation of a radio-frequency generator, in particular a radio-frequency generator from the group of half-bridge invertor, full-bridge invertor and push-pull invertor.
The invention""s method for generating digital control signals has the following method steps:
A) storage of a predeterminable value of a first variable in the first storage means,
B) feeding of the value of the first variable stored in the first storage means into the adder as first summand,
C) feeding of an instantaneous value of a second variable stored in the second storage means into the adder as second summand,
D) determination of the sum of the first and second summands in the adder and transfer of a carry signal to the pulse divider,
E) storage of the addition result determined by the adder in the second storage means as new instantaneous value of the second variable,
F) repetition of the abovementioned method steps B) to F) in a cycle predetermined by the oscillator.
With the aid of the working method described above, digital control signals that can be used for pulse-width-modulated driving are generated whose frequency can be gradated in comparatively small steps. Given a predetermined oscillator frequency and predetermined hardware properties, the frequency of the digital control signals generated by the method according to the invention is determined by the specification of a numerical value for the first variable. A change in frequency of the digital control signals generated by the method according to the invention is advantageously brought about by changing the value of the first variable stored in the first storage means and executing method steps A) to F).